1. Field of the Invention
The present invention relates to high tensile strength steels suitable for use in civil engineering, construction, bridges, marine structures, pipes, reservoirs, construction equipment, etc. More particularly, the invention relates to a high tensile strength steel product for high heat input welding having excellent toughness in the heat-affected zone, and preferably having a tensile strength of 490 MPa or more.
2. Description of the Related Art
Recently, as the size of structures has increased, thick steel products having high strength have been increasingly used. Moreover; in order to reduce construction costs, high heat input welding with a high weld efficiency has been employed. However, when the amount of heat input is increased, embrittlement in the weld zone may occur. Therefore, steel products suitable for high heat input welding, in which the toughness in the high heat input weld zone is improved, have been desired, and various proposals have been made.
For example, Japanese Unexamined Patent Application Publication No. 58-31065 discloses a technique for improving the heat-affected zone (HAZ) toughness by suppressing the coarsening of austenite grains using a nitride, for example, TiN. However, in certain zones, such as the weld bond zone, which are heated at high temperatures, the nitride, such as TiN, is dissolved and its capacity for suppressing grain coarsening is lost. Therefore, in the method in which nitride such as TiN is used, the toughness in the weld bond zone does not greatly improve, and, in particular, it is difficult to improve the toughness in the HAZ when it is subjected to very high heat input welding in which the heat input exceeds 80 kJ/mm.
Japanese Unexamined Patent Application Publication No. 60-245768 also discloses a method for improving the HAZ toughness by accelerating the precipitation of intragranular ferrite using titanium oxides or a complex of titanium oxides and titanium nitride. In accordance with this method, the grain coarsening can be suppressed by the pinning effect of the oxides which do not dissolve even at high temperatures. However, it takes advanced steelmaking techniques to disperse the titanium oxides homogeneously in steels, so that it is very difficult to produce such steel products stably and in large quantity. Additionally, since the melting points of the titanium oxides are as high as approximately 1,700xc2x0 C. or more, nozzle clogging may readily occur due to the adhesion to the nozzle wall.
Japanese Unexamined Patent Application Publication No. 5-186848 also discloses a method for forming a HAZ having excellent toughness in which a compound precipitate of TiNxe2x80x94MnSxe2x80x94VN is dispersed in steels by adjusting the combined contents of C, V, and N with the addition of Ti, using the intragranular ferrite nucleation capacity of VN. However, since the effect of suppressing the coarsening of austenite grains by TiN is lost in very high heat input welding, the toughness in the HAZ which is subjected to very high heat input welding cannot be improved, and also the addition of MnS for accelerating the precipitation of VN decreases the cleanliness of steels and deteriorates the toughness of the base metal.
Accordingly, it is an object of the present invention to provide a high tensile strength steel product having excellent toughness in the high heat input weld zone, in which the toughness of the weld zone is not decreased even if high heat input welding with a heat input of more than 80 kJ/mm is performed.
The present inventors have made extensive efforts and carried out exhaustive research to develop a method of finely and homogeneously dispersing oxide inclusions, so as to refine austenite grains in the weld zone and to accelerate the precipitation of intragranular ferrite. As a result, it has been found that in order to finely and homogeneously disperse oxide inclusions which are effective in refining austenite grains and accelerating the precipitation of intragranular ferrite, the oxide inclusions must contain titanium oxides as a principal ingredient and the oxide inclusions must be within optimum compositional ranges.
Initially, the results of that research as regards the optimum ranges of the oxide inclusions will now be described.
First, in order to achieve a fine, homogeneous dispersion of oxide inclusions, satisfactory wettability is required between deoxidizing inclusions and molten steel, and for that purpose, the Al2O3 content in the total inclusion must be reduced to at most about 70% by weight.
Second, in order to accelerate the precipitation of intragranular ferrite, the titanium oxide content in the total oxide inclusion must be set to at least about 20% by weight; the MnO content in the total oxide inclusion must be at most about 15% by weight; and the content of CaO or a rare earth metal (REM) oxide in the total oxide inclusion must be at most about 50% by weight.
Third, in order to prevent nozzle blocking, the melting point of deoxidation products must be lowered, and for that purpose, the content of CaO or the REM oxide in the total inclusion must be at least about 5% by weight by Ca treatment or REM treatment; and also, the Al2O3 content and the titanium oxide content must be at most about 70% by weight and at most about 95% by weight, respectively.
Based on the findings described above, the optimum compositional ranges for the oxide inclusions have been determined by the present inventors. That is, as shown in FIG. 1, the Ti oxide content is in the range of from about 20% to about 95% by weight, the content of at least one of CaO and a REM oxide is in the range of from about 5% to about 50% by weight in total, and the Al2O3 content is at most about 70% by weight. Additionally, the MnO content is at most about 15%. By controlling the composition of the oxide inclusions in the ranges shown in FIG. 1, without causing nozzle clogging or the formation of harmful inclusion clusters, the intragranular ferrite formation capacity of the inclusions can be effectively used.
The present inventors have discovered that titanium oxides not only themselves act as sites for ferrite nucleation but also act as sites for precipitation of MnS and VN, which also possess intragranular ferrite formation capacity. The present inventors have also discovered that, in order to further accelerate the precipitation of intragranular ferrite, in addition to the fine, homogeneous dispersion of the oxide inclusions, incorporating V and N as ingredients of steel forms a compound precipitate, such as that which contains a titanium oxide and VN, and thus the intragranular ferrite formation capacity is significantly increased.
The present invention has been achieved based on the knowledge and discoveries described above.
That is, a high tensile strength steel product for high heat input welding having excellent toughness in the heat-affected zone and having a tensile strength of at least about 490 MPa, in accordance with the present invention, contains, in terms of percent by weight, from about 0.05% to about 0.18% C, at most about 0.6% Si, from about 0.80% to about 1.80% Mn, at most about 0.005% Al, at most about 0.030% P, at most about 0.004% S, at most about 0.005% Nb, from about 0.04% to about 0.15% V, from about 0.0050% to about 0.00150% N, and from about 0.010% to about 0.050% Ti, the ratio of the Ti content to the Al content (Ti/Al) being at least about 5.0; and further contains at least one of (a) from about 0.0010% to about 0.0100% of Ca and (b) from about 0.0010% to about 0.0100% of REM, balance Fe and incidental impurities. In the steel product, oxide inclusions are dispersed, which contain, in terms of percent by weight, from about 20% to about 95% titanium oxide, at most about 70% Al2O3, from about 5% to about 50% in total of at least one of calcium oxide and a REM oxide, and at most about 15% MnO.
Preferably, the steel product further contains, in terms of percent by weight, at least one of the following ingredients, in the following amounts: from about 0.05% to about 1.0% Cu, from about 0.05% to about 0.50% Ni, from about 0.05% to about 0.50% Cr, and from about 0.02% to about 0.20% Mo. Preferably, the steel product further contains, in terms of percent by weight, from about 0.0005% to about 0.0030% B.